Aerial lifts are frequently employed for lifting operatives to elevated working sites, for example to install overhead pipe work during building construction. A typical aerial lift may comprise a mobile elevated work platform (MEWP) having an extendable boom which has an elevator basket or cage for housing operatives securely to the end of the boom. Alternative aerial lifts may comprise a MEWP having an extendable scissor lift which has an elevator basket or cage for housing operatives securely to the end of the scissor lift.
The basket or cage generally contains a control panel which permits an operative standing in the basket or cage to manoeuvre the work platform as desired. The aerial lift may be powered using hydraulics powered by the MEWP's engine.
The control panel generally features an emergency stop switch and a function enable switch which needs to be operated before the controls can be operated. In boom lifts the function enable switch is typically a footswitch which must be depressed to activate the controls. In scissor lifts the function enable is typically a dead man's handle. If the function enable switch is released, the basket or cage is prevented from moving immediately, but the MEWP's engine (which powers the hydraulics) continues to run. If the emergency stop switch is activated then both the basket is prevented from moving and, in most cases, the MEWP's engine is stopped.
Unfortunately it is known that operators standing at the control panel of the aerial lift can become trapped between the basket and an obstacle before they can either release the function enable switch or activate the emergency stop. This is known as an entrapment event. Accidents of this nature can be fatal since the operator can be crushed.
The present applicants have previously described a safety device comprising a tensioned cord or wire (EP2096078B1) and an improved system comprising a pressure sensitive safety edge (WO2012/001353). Activation of the safety device, which is located proximate to the control panel, prevents movement of the basket or cage by overriding the function enable switch. The effect being equivalent to the operative having released the function enable switch. A further improvement was disclosed in WO2013/093395 whereby the safety device is not “live” until the operator activates the function enable switch.
In each of the prior art safety devices it is possible for the relays in the function enable switch to become “welded”. That is, they do not respond to the safety device signal, thereby rendering the safety device non-functional. Clearly, this represents a danger to the operator.
The present applicants have overcome this problem by employing a backup safety system in the safety device of the present disclosure.